Process for urea synthesis

ABSTRACT

A PROCESS FOR THE HIGH PRESSURE SYNTHESIS OF UREA FROM AMMONIA AND CARBON DIOXIDE IS PROVIDED, IN WHICH RECYCLE AQUEOUS AMMONIUM CARBAMATE SOLUTION IS PRESSURIZED AND STRIPPED WITH FEED CARBON DIOXIDE, PREFERABLY AT OR SLIGHTLY ABOVE UREA SYNTHESIS REACTOR PRESSURE. THE STRIPPING STEP IS CARRIED OUT IN A HEAT EXCHANGE ZONE WHICH IS EXTERNALLY HEATED TO DECOMPOSE AMMONIUM CARBAMATE, SO THAT A HIGH PRESSURE OFF-GAS PRINCIPALLY CONTAINING CARBON DIOXIDE AND AMMONIA IS PRODUCED FROM THE STRIPPING STEP, TOGETHER WITH A LIQUID EFFLUENT PRINCIPALLY CONSISTING OF WATER, WHICH MAY BE DISCARDED OR PROCESSED TO RECOVER AMMONIA VALUES. FEED AMMONIA IS PREFERABLY ADDED TO THE HIGH PRESSURE OFF-GAS FROM STRIPPING, TO PRODUCE A COMBINED PROCESS STREAM HAVING AN AMMONIA TO CARBON DIOXIDE MOLAR RATIO OF ABOUT 2:1, WHICH IS PASSED THROUGH A DECOMPOSER-HEAT EXCHANGE ZONE TO HEAT EFFLUENT FROM THE UREA SYNTHESIS REACTOR, THUS DECOMPOSING AMMONIUM CARBAMATE IN THE UREA SYNTHESIS EFFLUENT AND PRODUCING AN OFF-GAS WHICH IS PROCESSED TO PRODUCE THE AQUEOUS AMMONIUM CARBAMATE SOLUTION. THE COMBINED PROCESS STREAM MAY COOL DURING CONDENSATION, DEPENDING ON THE AMMONIA TO CARBON DIOXIDE RATIO IN THE DECOMPOSER-HEAT EXCHANGE ZONE, HOWEVER THE PRINCIPAL EFFECT ON THE COMBINED PROCESS STREAM IS CONDENSATION OF MOLTEN AMMONIUM CARBAMATE, TO PRODUCE A GAS-LIQUID MIXTURE OF LOW OR NEGLIGIBLE WATER CONTENT. THE RESULTING PROCESS STREAM CONTAINING CONDENSED AMMONIUM CARBAMATE IS PASSED TO UREA SYNTHESIS. IN AN ALTERNATIVE EMBODIMENT OF THE INVENTION, THE COMBINED PROCESS STREAM PRODUCED BY ADDING AMMONIA TO THE STRIPPER OFF-GAS MAY BE COOLED IN HEAT EXCHANGE WITH LIQUID WATER WHICH IS VAPORIZED TO PRODUCE STEAM, WHICH MAY BE EMPLOYED IN A SEPARATE UNIT TO HEAT THE UREA SYNTHESIS EFFLUENT AND DECOMPOSE AMMONIUM CARBAMATE.

J- F. VILLIERS-FISHER E AL 3,636,106

PROCESS FOR UREA SYNTHESIS Filed Sept. 12, 1968 JOHN F. VlLLlERS-FISHERPHILIP F. KAUPAS INVENTORS.

AGENT Jan. 18, 1972 52; mm 2238 292 1520 mm wZEmmS f r 2220253 @503@030: EIESTR mlto 852 on r mm m mm mm M5205 zommfi N Em: m H R. mm 9 5mmm ES; H3325 $539; \0@ m mm me wm v w m m mwmzmazou 581283 20$ S N 29:33M22513 222023 26%? ww G mm United States Patent 3,636,106 PROCESS FORUREA SYNTHESIS John F. Villiers-Fisher, Kendall Park, and Philip F.

Kaupas, Old Bridge, N.J., assignors to Chemical ConstructionCorporation, New York, N.Y.

Filed Sept. 12, 1968, Ser. No. 759,371 Int. Cl. C07c 127/00 U.S. Cl.260555 A 28 Claims ABSTRACT OF THE DISCLOSURE A process for the highpressure synthesis of urea from ammonia and carbon dioxide is provided,in which recycle aqueous ammonium carbamate solution is pressurized andstripped with feed carbon dioxide, preferably at or slightly above ureasynthesis reactor pressure. The stripping step is carried out in a heatexchange zone which is externally heated to decompose ammoniumcarbamate, so that a high pressure off-gas principally containing carbondioxide and ammonia is produced from the stripping step, together with aliquid efiiuent principally consisting of water, which may be discardedor processed to recover ammonia values. Feed ammonia is preferably addedto the high pressure oE-gas from stripping, to produce a combinedprocess stream having an ammonia to carbon dioxide molar ratio of about2: 1, which is passed through a decomposer-heat exchange zone to heatefiluent from the urea synthesis reactor, thus decomposing ammoniumcarbamate in the urea synthesis efiiuent and producing an off-gas whichis processed to produce the aqueous ammonium carbamate solution. Thecombined process stream may cool during condensation, depending on theammonia to carbon di oxide ratio in the decomposer-heat exchange zone,however the principal effect on the combined process stream iscondensation of molten ammonium carbamate, to produce a gas-liquidmixture of low or negligible water content. The resulting process streamcontaining condensed ammonium carbamate is passed to urea synthesis. Inan alternative embodiment of the invention, the combined process streamproduced by adding ammonia to the stripper off-gas may be cooled in heatexchange with liquid water which is vaporized to produce steam, whichmay be employed in a separate unit to heat the urea synthesis eflluentand decompose ammonium carbamate.

BACKGROUND OF THE INVENTION Field of the invention The invention relatesto recycle-type urea synthesis processes in which urea is produced bythe reaction of ammonia and carbon dioxide at elevated pressure, toproduce a urea synthesis efliuent containing urea, water, ammoniumcarbamate and usually containing excess ammonia, and in which theoff-gas from heating of the urea synthesis effluent to vaporizeunreacted ammonium carbamate and ammonia is processed to produce anaqueous ammonium carbamate solution for recycle to urea synthesis.

Description of the prior art Urea is synthesized from ammonia and carbondioxide, typically at 210 kg./ sq. cm. and 190 C., using an overallammonia to carbon dioxide mole ratio of about 4:1. The reaction proceedsthrough the initial formation of ammonium carbamate, a portion of whichis dehydrated to yield urea. The synthesis reactor eflluent thuscontains urea, water, ammonium carbamate and excess ammonia. The reactoreffiuent is heated at reduced pressure to decompose ammonium carbamateand produce a mixed off-gas containing ammonia, carbon dioxide and watervapor, together with product aqueous urea solution, which is furtherprocessed to produce solid urea crystals or prills. A part of theunreacted mixed oii-gas is recycled in an aqueous ammonium carbamatesolution having a water content such that the feed ratio of water tocarbon dioxide in the overall synthesis reactor feed is about 0.6 to 1on a molar basis. Since water is one of the products of the ureasynthesis reaction, the synthesis conversion to urea is reduced from thetheoretical value of about 77% to about 67%.

The pressure of the reactor efiiuent is reduced, typically to 21 kg./sq. cm., and the efiiuent is then heated to about 160 C. About of theunreacted ammonia and carbon dioxide are evaporated at 21 kg./ sq. cm.Off-gas may be generated by adiabatic flash evaporation due to pressurereduction, as described in U.S. Pat. No. 3,172,911 and U.S. patentapplication No. 521,921, filed Jan. 20, 1966 and issued as U.S. Pat. No.3,527,799. Some energy is recovered during the condensation of thevapors, typically by the procedures described in U.S. Pats. Nos.3,147,304 and 3,137,725. Typical usages for the energy are as a heatsource for second stage decomposition at about 2.1 kg./ sq. cm., forheating the urea solution in the crystallizer, or for feed ammoniapreheat.

The partially condensed vapors or off-gas from the 21 kg./sq. cm.evaporation or carbamate decomposition and the off-gas from the 2.1kg./sq. cm. evaporation are collected in water in two absorbers andrecycled to the reactor as concentrated aqueous ammonium carbamatesolution. Much of the ammonia from the evaporation stages, especiallythe first stage evaporation at 21 kg./sq. cm., is stripped of carbondioxide in a tower and condensed as nearly pure ammonia. Off-gastreatment procedures are described in U.S. Pats. Nos. 3,155,722;3,155,723 and 3,191,916.

The stripping of unreacted components from the urea synthesis efiluentemploying ammonia or carbon dioxide as the stripping medium, so as toproduce a stripped product aqueous urea solution, is described in U.S.Pats. Nos. 2,056,283; 3,046,307; 3,049,563; 3,301,897 and 3,356,725 andCanadian Pat. No. 787,960. The use of mixed oifgases for stripping isdescribed in Canadian Pat. No. 736,- 520 and U.S. Pat. No. 3,072,721;and the use of inert gas or air for stripping is described in US. Pats.Nos. 2,267,- 133; 2,087,325 and 3,120,563.

SUMMARY OF THE INVENTION The stripping process of the present inventionis applicable to any urea process using a recycle aqueous ammoniumcarbamate solution. Energy reuse techniques as practiced in the presentprocess scheme may change significantly with changes in the ammonia tocarbon dioxide ratios in the reactor feed. The energy reuse techniquescontrol the overall energy requirements. The features of the presentinvention include the combination of high pressure carbon dioxidestripping of aqueous ammonium carbamate solution, with the condensationof the resulting mixed oli-gas to ammonium carbamate melt at highpressure, to heat or drive other parts of the process. The process ofthe invention features the selection and high pressure stripping of acarbamate-bearing, urea-free process stream which is relativelyinsensitive to process conditions. Thus, the present invention avoidsthe problems attendant upon stripping of the urea bearing solutions,where residence at the temperatures required and at reactor pressures iscritical, with respect to urea hydrolysis and/ or biuret formation.

In the present invention, the recycle aqueous ammonium carbamatesolution typically derived from the 4:1 liquid recycle urea process isstripped of ammonium carbamate and ammonia employing the carbon dioxidefeed stream, preferably at or somewhat above reactor pressure and attemperatures such that relatively complete stripping can be achieved.The pressure of the steam inthe heating steam jacket disposed about thestripping zone should generally exceed 14 kg./sq. cm. and is limitedonly by the economics of jacket construction and corrosion of thestripper as the wall temperature exceeds 190 C. Presently practicalsteam pressures are kg./ sq. cm. up to the 40 kg./sq. cm. steamavailable from the usual associated ammonia plant.

The overhead vapors are adjusted to near the maximum condensationtemperature mixture, which is roughly 2 mols ammonia per mol of carbondioxide, by ammonia addition. In general, the major portion of thevapors are condensed before going to the reactor. The heat ofcondensation is preferably employed in part to decompose ammoniumcarbamate in the reactor efiiuent and evaporate carbon dioxide, ammoniaand some water from the reactor effluent at reduced pressure, thepreferred pressure range being about 15 kg./sq. cm. to about 55 kg./sq.cm. The residual excess heat derived from carbamate condensation in theoverhead vapor stream, above the requirement for decomposition ofammonium carba-mate in the reactor efiluent, is employed to generate 3to 6 kg./sq. cm. steam. Alternatively, a major portion of the heat ofcarbamate condensation could be converted to steam, with part of thesteam being used to decompose ammonium carbamate in the reactorefiiuent.

The stripper liquid underfiow, consisting mostly of water, willgenerally contain some ammonia and carbon dioxide, with the quantitydepending on local economies. This material is recovered by distillationin a still. In a preferred embodiment of the invention, all the Waterremoved from the product aqueous urea solution is condensed and isemployed to absorb all of the ammonia and carbon dioxide removed fromthe urea synthesis efiluent at low pressure. This solution is alsopassed to the same still for distillation. In an alternative, the highpressure stripper bottoms can be stripped in the high pressure unit tothe point where only carbon dioxide and water remain in the liquidphase, and the efiluent is then discarded. In any case, the stilldistillation overhead passes ultimately to the high pressure absorberwhich produces the aqueous ammonium carbamate solution.

A significant part of the heat content of the mixed olfgas or vaporsderived from the decomposition of ammonium carbamate in the reactoreffluent is recovered by heating the high pressure ammonia feed to thereactor. These vapors and/ or the still vapors mentioned supra can alsobe used to provide the heat requirements for the urea crystallizer.These vapors can also be used as a heat source in a low pressurestripper, to remove the residual ammonia in the reactor efiluent.

The admixing of the high pressure stripper overhead with ammonia can beused to control the extent of carbamate decomposition and evaporation inthe reactor eifiuent. By adjusting the ammonia feed rate to move themolar ratio away from 2:1, the condensation temperature is reduced. Thislowering of the temperature driving force reduces the amount ofcarbamate decomposition in the reactor efiluent and cocomitantevaporation or generation of mixed off-gas. This is an advantage whenthis decomposer is sized for the fouled condition but is on line in anearly clean condition.

In an alternative embodiment of the invention, the high pressurestripping step can be modified by admixing enough carbon dioxide intothe aqueous ammoniacal ammonium carbamate solution to react with same ofthe free ammonia in solution and heat solution to feed traytemperatures. The heat of reaction due to a further excess of carbondioxide addition and ammonium carbamate formation could be absorbed inheat exchange with the ammonia feed stream. The end result is improvedsystem performance, but with higher high pressure steam consumption anda higher export of low pressure steam. This modification may bejustified on the bases of the local market for such low pressure steam,which may be employed as a heat source for solution heating or the like.

The process of the present invention provides numerous advantages. Thereduction in the Water content of the recycled ammonium carbamate, andconsequent reduction in the Water level during urea synthesis, raisesthe reactor conversion from 67% to 75%. This reduces the energyrequirement for evaporation about 25%.

The equipment for the high pressure stripping process is sized to alarge extent on the basis of the available temperature driving force.The advantage of processing the recycle aqueous ammonium carbamatesolution in the stripper is that the film temperatures can be muchhigher than when processing the reactor efliuent since ureadecomposition is not a problem. Presently the allowable steam pressureis limited to about 35 kg./sq. cm. because of corrosion of titanium orstainless steel by the ammonium carbamate. However, modified titanium orstainless or other alloys could permit operation with much hotterheating media, the choice being primarily based on economicconsiderations.

The stripper operating pressure is variable over a wide range. Theoperating pressure is preferably slightly above reactor pressure, inorder to avoid the necessity of pumping hot condensed ammoniumcarbamate, to eliminate any restrictions on both the stripper and thecondenser locations since pressure is available to drive the liquid tothe reactor, and to minimize the stripper skin temperature to permit theuse of alloys such as 316 L. The condensation temperature perm-its thegeneration of 7 kg./ sq. cm. steam. Higher operation pressures arepossible, particularly with higher stripper jacket temperatures, andsteam regeneration may take place at much higher pressures. Reprocessingthe recycle carbamate solution therefore lends itself to the multiplereuse of energy. At very high condensing temperatures, liquid residencemust be minimized to avoid byproduct formation beyond the urea stage.

In most instances, direct transfer of heat 'will be provided between thecondensing ammonium carbamate and the reactor effiuent in a heatexchanger. This relationship may be modified, for instance steam couldbe generated first and then the generated steam could be used toevaporate the ammonia and carbon dioxide from the reactor effiuent. Inthis latter step, effiuent liquor residence in the exchanger, filmtemperature and free ammonia partial pressures define to a large extentthe biuret formation and urea hydrolysis ratio. Consequently, at highcarbamate condensing temperatures, the intermediate generation of steammay be preferred as an alternative to the design of a special condensingcarbamate-reactor efliuent heat exchanger.

The flexibility in ammonium carbamate condensing temperatures simplifiesthe design of the higher pressure evaporation and absorption step, sincehigh reuse temperatures permit equivalent evaporation at high pressures,which simplifies the second reuse of the high pressure steam and theultimate condensation of the excess ammonia. A final advantage of therecycle of nearly dry molten ammonium carbamate is a significantreduction in reactor volume.

It is an object of the present invention to provide an improved processfor the synthesis of urea.

Another object is to provide an improved process for the recycle ofaqueous ammonium carbamate solution to urea synthesis.

A further object is to dehydrate aqueous ammonium carbamate solution inan improved manner, prior to recycle to urea synthesis.

An additional object is to utilize high pressure feed carbon dioxide inan improved manner, to strip aqueous ammonium carbamate solution forrecycle to a urea synthesis process.

Still another object is to reduce the proportion of water recycled to aurea synthesis process in which mixed origas is processed to produce anaqueous ammonium car bamate solution.

Still a further object is to provide an improved heat source fordecomposition of ammonium carbamate in a urea synthesis reactorefiluent.

An object is to strip aqueous ammonium carbamate solution at highpressure with feed carbon dioxide in a urea synthesis process, andthereby eliminate water from the system and produce a high pressure gasstream which, when adjusted to a suitable ammonia to carbon dioxidemolar ratio by ammonia addition, is usable as a high temperature heatsource via ammonium carbamate condensation at high pressure.

An object is to increase conversion in the urea synthesis reactor byreducing the water content of the recycle ammonium carbamate solution.

An object is to separate ammonia and carbon dioxide from the aqueousammonium carbamate solution by stripping with carbon dioxide atpressures such that the overhead mixed gas stream condenses at usefultemperature levels for energy reuse, preferably at or above reactorpressures.

These and other objects and advantages of the present invention willbecome evident from the description which follows.

DESCRIPTION OF THE DRAWING AND PREFERRED EMBODIMENTS Referring now tothe drawing, a fiowsheet of a preferred embodiment of the process of theinvention is presented. Preheated ammonia feed stream 1 and recyclestream 2 containing ammonium carbamate at least partially in the liquidstate are passed into urea synthesis reactor 3, which is generally ahigh pressure autoclave or the like. An elevated pressure generally inthe range of 100 kg./sq. cm. to 350 kg./sq. cm. and preferably in therange of 135 kg./sq. cm. to 270 kg./ sq. cm. is maintained within unit3, together with an elevated temperature generally in the range of 150C. to 220 C. The overall molar feed ratio of ammonia to carbon dioxideis maintained within unit 3 in a range generally between 2:1 and about8: 1, and under these operating conditions the synthesis of urea bydehydration of ammonium carbamate takes place within unit 3. Thedehydration reaction reaches equilibrium and total conversion to urea isnot obtained in unit 3 in commercial practice, and consequently theeffiuent stream 4 discharged from the urea synthesis reactor or zone 3contains urea, water, ammonium carbamate and excess ammonia. Stream 4 ispassed through pressure reducing valve 5, and the resulting effluentstream 6, which is now at a reduced pressure below 100 kg./sq. cm. andgenerally in the range of 15 kg./sq. cm. to 55 kg./sq. cm., is passedinto the shell of heat exchanger-decomposer 7 and rises external to thetubes 8, which may be provided with suitable external baflies. Thesynthesis effluent process stream is heated in the shell of unit 7, anda portion of the ammonium carbamate in the process stream is decomposed,with the resultant formation of a gaseous phase containing ammonia,carbon dioxide and water vapor.

The resulting gas-liquid mixture formed in the shell of unit 7 isdischarged from the upper portion of unit 7 via stream 9, which passesto gas-liquid separator 10 for separation of the gaseous phase from theresidual urea-containing liquid phase. Unit 10 is a gas-liquid separatorof conventional design, and may consist of a baffied or cyclonic unit.The separated gaseous phase is removed from unit 10 via stream 11, whichis processed for the formation of an aqueous ammonium carbamate solutionin a manner to be described infra. The residual urea-containing liquidphase is removed from unit 10 via stream 12, which is passed throughpressure reducing valve 13, which discharges the resulting stream 14 ata reduced pressure in the range of 1 kg./sq. cm. to kg./sq. cm.

Due to the reduced pressure of stream 14, a gaseous phase is evolved instream 14, which is separated into gaseous and residual liquid phases bypassing stream 14 into gasliquid separator 15. Unit 15 is similar tounit 10 described supra, and separates the evolved off-gas phase stream16 from the residual liquid phase stream 17. Stream 16 contains ammoniaand carbon dioxide together with a small proportion of water vapor, andis processed for recycle and recovery of ammonia values as will be described infra.

Stream 17 now consists primarily of an aqueous urea solution containingminor dissolved proportions of ammonia and carbon dioxide, and is nowprocessed in any suitable manner to recover product solid urea. Stream17 is preferably combined with recycle stream 18 which is derived in amanner to be described infra, and the resulting combined stream 19 ispassed into vacuum crystallizer 20 for the formation of solid ureacrystals by the removal of water as vapor, together with residualamounts of ammonia and carbon dioxide. The resulting slurry stream 21removed from the bottom of unit 20 now consists of solid urea crystalsentrained in a saturated aqueous mother liquor solution. Stream 21 isnow preferably divided into two portions, with recycle portion stream 22being heated in heat exchange 23 and recycled via stream 18. The balanceof stream 21 passes via stream 24 to centrifuge or crystal filter 25,from which product solid urea crystals stream 26 passes to productutilization. In some cases stream 26 may be passed directly to productusage as a fertilizer, in plastics manufacture, or the like, howeverstream 26 may alternatively be dried, melted and prilled to form ureaprills for fertilizer usage. The residual mother liquor stream 27removed from unit 25 may be recycled to unit 20 or further utilized inthe process as will appear infra. In some instances, stream 21 may beinitially processed to produce a dense slurry which is utilized asstream 24, and a clear liquor which is recycled via stream 22.

Returning to unit 20, an overhead vapor phase stream 28 principallycontaining water vapor together with residual ammonia and carbon dioxideis inducted into steam jet exhauster 29, through which stream 30 ispassed and expanded to induct stream 28 and provide a vacuum in unit 20.The resulting combined stream 31 discharged from unit 29 is cooled andmay be at least partially condensed in heat exchanger 32, which isprovided with cooling water or other suitable coolant. The resultingcooled stream 33 is combined with stream 16 in cooler-condenser 34,which condenses all or a major portion of the combined streams to theliquid state. The resulting liquid stream 35 is processed to recoverammonia values as will appear infra.

Returning to unit 10, the overhead mixed off-gas stream 11 is combinedwith recycle stream 36, which is derived in a manner to be describedinfra and contains ammonia, carbon dioxide and water, with thesecomponents being present in stream 36 at least partially in a condensedliquid phase. The resulting hot combined stream 37, which is elevated intemperature due to the formation of ammonium carbamate in the liquidphase, is cooled and partially further condensed to the liquid state inheat exchanger 38. The resulting mixed gas-liquid stream 39 now passesinto condenser-stripper unit 40, which is an apparatus and has afunction similar to these described in US. Pats. Nos. 3,155,723 and3,191,916. The liquid portion of stream 39 passes downwards and joinsthe liquid body in the bottom portion of unit 40, while the gaseousportion of stream 39 flows upwards through gas scrubbing andcondensation section 41, which is typically a bed of suitable spherical,ring or saddle-type packing for efficient gas-liquid contact. In someinstances, section 41 may consist of sieve trays, bubble cap plates orthe like. In any case, cold liquid stream 42 consisting of an aqueousammoniacal ammonium carbamate solution is passed into unit 40 abovesection 41, and flows downwards through section 41 countercurrent to therising gas phase, thus serving to scrub dioxide from the gas phase as acondensed ammonium carbamate. The resulting aqueous ammonium carbamatesolution which collects in the bottom of unit 40 and usually containsresidual free ammonia is withdrawn via stream 43. A portion of thestream 43 is recycled via stream 42, which may be cooled in an externalheat exchanger, not shown, prior to recycle to unit 40 above section 41.

The balance of stream 43, consisting of stream 44, is recycled to ureasynthesis in accordance with the present invention. Stream 44 is pumpedand pressurized in pump 45, which pressurizes the aqueous ammoniumcarbamate solution to an elevated pressure which is usually slightlygreater than the urea synthesis pressure in unit 3, and in any case isgenerally in the range of about 30 kg./sq. cm. to 350 kg./sq. cm., andpreferably in the range of 100 kg./sq. cm.to 350 kg./sq. cm. Theresulting pressurized stream 46 discharged from unit 45 is now passed tostripper-heat exchanger 47 for the removal of ammonium carbamate by highpressure stripping with feed carbon dioxide, which removes water fromthe system and pro duces a high pressure mixed gas stream in accordancewith the present invention.

Stream 46 flows downwards through the tubes 48 in unit 47. The tubes 48are externally heated, typically by passing a hot fluid stream 49 whichusually consists of high pressure steam external to the tubes 48, withcooled fluid or condensed water being removed via stream 50. Theexternal heating of tubes 48 promotes the internal decomposition ofammonium carbamate derived from stream 46 and the formation of a gaseousphase containing ammonia and carbon dioxide at elevated pressure withinthe tubes 48. A temperature typically in the range of about 200 C. toabout 250 C. is maintained within unit 47 by stream 49. Feed carbondioxide gas is introduced into the bottom of unit 47, tyipcally bycompressing feed carbon dioxide gas stream 51 in compressor 52 to anelevated pressure, usually slightly higher than urea synthesis pressureand generally comparable to the pressure of stream 46, and passing theresulting compressed carbon dioxide gas stream 53 into the bottom ofunit 47, so that the feed carbon dioxide gas rises through the tubes 48and strips ammonia and carbon dioxide from the downflowing aqueousliquid solution within tubes 48. A residual aqueous phase consistingmostly of water collects in the bottom of unit 47, and is withdrawn viastream 54. =In some instances, stream 54 may have a negligible contentof dissolved ammonia and is discharged to waste. However, in thispreferred embodiment of the invention, stream 54 is further processed torecover dissolved ammonia and carbon dioxide values.

Stream 54 is passed through pressure reducing valve 55, an the resultingaqueous stream 56, now at a reduced pressure typically in the range ofkg./ sq. cm. to 55 kg./ sq. cm, is passed into ammonia recovery still 57together with stream 35, which was produced from processing as describedsupra. In most instances, stream 35 as produced is at a pressure below15 kg./ sq. cm., and therefore stream 35 is pressurized by passingthrough a pump, not shown, prior to flowing into unit 57. Still 57 is adistillation column or still, which boils and refiuxes the vapors fromthe aqueous feed streams 56 and 35 at a pressure typically in the rangeof 15 kg./sq. cm. to 55 kg/sq. cm. and temperature in the range of 150C. to 230 C., in order to separate an overhead vapor stream rich inammonia from waste water. The depleted waste water stream 58 iswithdrawn from the bottom of unit 57, and a portion of stream 58 isdiscarded as discharged waste water stream 59. In some instances, heatmay be recovered from hot Water stream 59 by passing stream 59 in heatexchange with stream 35, prior to final discharge of stream 59 to waste.The balance of stream 58 is passed via stream 60 to r'eboiler-heatexchanger 61 for heating and partial vaporization in heat exchange withsteam, and the resulting mixed vapor-liquid stream 62 is recycled tounit 57 below the upper reflux trays section. A final hot overhead vaporstream 63 is removed from the top of unit 57. Stream 63 is rich inrecovered ammonia values, and also contains carbon dioxide and watervapor. Stream 63 is initially passed through heat exchanger 23 and inheat exchange with stream 22 is described supra. At least a portion ofstream 63 condenses to liquid in unit 23, and the resulting cooledstream 36 containing a condensed liquid phase is combined with stream.11 and recycled to further proc essing as described supra, forconversion to aqueous ammonium carbamate solution and purified overheadammonia vapor in unit 40.

Returning to condenser-stripper unit 40, the rising gas phase abovesection 41 is now of depleted carbon dioxide content and consists mostlyof ammonia vapor. This gas phase rises through reflux-stripping section64, as described in the US. Pats. Nos. 3,155,723 and 3,191,916, and aresulting residual purified ammonia vapor phase stream 65 is withdrawnfrom the top of unit 40. Stream 65 is combined with feed ammonia stream66, and the combined ammonia stream 67 is preferably cooled and totallycondensed in heat exchanger 68 by heat exchange with cooling water. Theresulting liquid ammonia stream 69 is pressurized by pump 70 to anelevated pressure typically in the range of 100 kg./sq. cm. to 350kg./sq. cm., and the pressurized liquid ammonia stream 71 dischargedfrom unit 70 is heated in heat exchanger 38 by heat exchange withprocess stream 37. The heated high pressure ammonia stream 72 is nowpreferably divided into stream 1, which is passed to urea synthesis asdescribed supra, and stream 73, which is utilized in accordance with thepresent invention.

Stream 73 is now preferably combined with hot mixed gaseous stream 74,which is derived from the tubes 48 of unit 47 as the resulting hot gasstream produced by the stripping of aqueous ammonium carbamate solutionwith gaseous carbon dioxide in unit 47. In most instances, theproportion of added ammonia stream 73 to mixed hot gas stream 74 will besuch that the resulting combined process stream 75 has a molar ammoniato carbon dioxide ratio in the range of about 1.5:1 to about 3:1, andfor optimum results in terms of subsequent maximum temperature levelsduring ammonium carbamate condensation, the molar ammonia to carbondioxide ratio in stream 75 is substantially 2:1. Urea stream 76 is nowpreferably added to stream 75, in order to promote the initialcondensation processes. In most cases, stream 76 will consist of aqueousurea solution, and may consist of a portion of streams 22 or 27.

The final combined recycle stream 77 produced by the addition of stream76 to stream 75 is now passed into unit 7 and flows downwards throughtubes 8. At least a portion of the ammonia and carbon dioxide content ofstream 77 condenses to molten ammonium carbamate of low water contentwithin tubes 8, with the consequent generation and liberation of heatwhich promotes the decomposition of ammonium carbamate in the ureasynthesis efiluent stream within the shell of unit 7, and describedsupra. The recycle process stream discharged downwards from tubes 8 iswithdrawn from unit 7 as stream 78, which usually contains both a moltenliquid ammonium carbamate phase and a residual gaseous phase.

Stream 78 is now processed for further heat recovery via carbamatecondensation, prior to recycle to urea synthesis. Stream 78 is passeddownwards into the vertically oriented heat exchanger 79, and flowsdownwards through tubes 80, in which further condensation of moltenammonium carbamate and heat liberation takes place. A suitable heatexchange fluid stream 81, which usually consists of condensate or boilerfeed water, is passed into the shell of unit 79, and heated heatexchange fiuid usually consisting of generated high pressure steam isremoved via stream 82. The external cooling of tubes 80 by the heatexchange fluid may promote and produce total condensation of the gaseousphase within tubes 80 to liquid. In any case, the resulting processstream discharged downwards from tubes 80 contains a molten liquidammonium carbamate phase and may consist entirely of molten ammoniumcarbamate of low water content. This resulting process stream collectsin unit 79 below tubes 80, and is withdrawn via stream 2 and passed tourea synthesis as described supra. In most instances, as mentionedsupra, the pressure of the process streams from which stream 2 isderived, specifically streams 53, 46 and 73, will be slightly higherthan the urea synthesis pressure in reactor 3, and consequently the hotmolten ammonium carbamate stream 2 will flow directly into reactor 3,without any necessity for the provision of a pump to force stream 2 intoreactor 3. This is highly advantageous, since a special pump for pumpinghot ammonium carbamate stream 2, which is a molten salt, is thereforenot required.

Numerous alternatives within the scope of the present invention, besidesthose mentioned supra, will occur to those skilled in the art. In someinstances, the invention may be practiced outside of some of the rangesof process variables such as temperature and pressure mentioned supra,however such ranges constitute preferred embodiments of the invention.Stream 14 may be heated prior to passing into unit 15, in order topromote final ammonium carbamate decomposition and generation of offgas,by heat exchange with streams 11 or 36. Stream 17 may alternatively beprocessed by multiple effect evaporation, to produce a substantiallyanhydrous urea melt for direct prilling. In this respect, the conceptsof US. Pats. Nos. 3,211,788 and 3,147,174 may be employed.

In some instances it will be advantageous to add carbon dioxide at highpressure to stream 46, since stream 46 usually contains excess freeammonia. In this case, the added carbon dioxide will react with the freeammonia in stream 46 to produce in situ formation of ammonium carbamatewith consequent temperature elevation. The resultant hot process streammay then be passed in heat exchange with stream 72, to provide furtherheating of feed ammonia, or may be passed directly into unit 47 atelevated temperature, thus reducing the heating requirements for unit47. In some instances the stripping pressure level in the tubes 48 ofunit 47 may be below urea synthesis reactor pressure. In this case,stream 74 may be compressed prior to combining with stream 73. In someinstances, such as when a high proportion of tree ammonia is present instream 46, stream 73 may be omitted and stream 75 Will be derived onlyfrom stream 74, however in most cases stream 73 will be provided inorder to produce the requisite molar ammonia to carbon dioxide ratio instream 75 for optimum temperature levels and heat generation. Asmentioned supra, the feed rate of stream 73 may be varied from thatwhich would provide the optimum 2:1 molar ratio of ammonia to carbondioxide in stream 75. This modification would be desirable duringinitial periods of operation when the heat exchange surfaces in units 7and 79 are clean and the tubes 8 and 80 are clear. As fouling occursduring sustained operating runs, and heat transfer rates become lowered,the feed rate of stream 73 relative to stream 74 may be modified toproduce a 2:1 molar ammonia to carbon dioxide ratio in stream 75, whichwould tend to promote subsequent ammonium carbamate condensation andheat release at maximum temperature levels.

Finally, the condensation of molten ammonium carbamate as practiced inthe present invention may be employed solely to generate steam or forother heat exchange purposes besides the heating of the urea synthesisefiluent stream 6. In this case, stream 77 would be passed directly intounit 79 and would bypass unit 7. The hot stream 82 generated in theshell of unit 79 and usually consisting of high pressure steam wouldthen be partly or totally employed for heating purposes in unit 7, andwould be passed through the tubes 8. This alternative is advantageouswhen portions of stream 77 initially condense to molten ammoniumcarbamate at very elevated temperatures, which could cause biuretformation and urea hydrolysis in the urea synthesis efiluent stream itdirect heat transfer from stream 77 to stream 6 is provided.

An example of an application of the present invention to the design of acommercial urea production facility will now be described.

EXAMPLE Following are the compositions and operating conditions forprincipal process streams in the process of the present invention, asapplied to the design of a commercial urea synthesis facility.Compositions are expressed in moles per mole of urea in the reactoreffluent stream 4.

Content of component, moles/mole urea Absolute Stream Carbon pressure,Temp No. dioxide Ammonia Water Urea kg./sq. cm. C.)

We claim:

1. In a process for the synthesis of urea from ammonia and carbondioxide in a urea synthesis zone at an elevated pressure in the range ofkg./sq. cm. to 350 kg./sq. cm. in which the pressure of the efiluentfrom the urea synthesis zone is reduced below 100 kg./ sq. cm., the ureasynthesis eflluent is heated at reduced pressure in a heat exchanger todecompose ammonium carbamate and generate a mixed off-gas containingammonia, carbon dioxide and water vapor, the residual liquid efiluent isseparated from said mixed off-gas and processed to recover product urea,and said mixed off-gas is at least partially condensed to produce anaqueous ammonium carbamate solution, the improved process for recycle ofammonium carbamate to urea synthesis which comprises (a) pressurizingsaid aqueous ammonium carbamate solution to an elevated urea synthesispressure in the range of 100 kg./sq. cm. to 350 kg./sq. cm.,

(b) passing said pressurized aqueous ammonium carbamate solution throughan externally heated stripping-heat exchange zone,

(c) passing a gaseous carbon dioxide stream through said stripping-heatexchange zone in contact with said aqueous ammonium carbamate solution,whereby ammonium carbamate is decomposed, ammonia and carbon dioxide arestripped from said solution into said gaseous carbon dioxide stream anda residual solution principally containing water is formed,

(d) removing said residual solution formed by step (c) from saidstripping-heat exchange zone,

(e) removing the resulting gaseous process stream formed by step (c)from said stripping-heat exchange zone, said resulting gaseous processstream containing ammonia and carbon dioxide components derived fromammonium carbamate decomposition in said zone, together with carbondioxide derived from said gaseous carbon dioxide stream,

(f) passing said resulting gaseous process tream through a heat recoveryzone in heat exchange with a fluid,

whereby at least a portion of said resulting gaseous process streamcondenses to liquid ammonium carbamate and thereby releases heat whichis absorbed by said fluid, and

(g) passing the resulting process stream discharged from said heatrecovery zone and containing condensed liquid ammonium carbamate to saidurea synthesis zone.

2. The process of claim 1, in which said stripping-heat exchange zone isvertically oriented, said pressurized aqueous ammonium carbamatesolution is passed downwards through said zone and said gaseous carbondioxide stream is passed upwards through said zone.

3. The process of claim 1, in which said residual solution principallycontaining water and removed according to step (d) also contains a minorproportion of dissolved ammonia, said residual solution is distilled toproduce a gaseous stream containing ammonia, and said gaseous stream isadded to said mixed off-gas generated by decomposition of ammoniumcarbamate in said urea synthesis effluent, prior to processing saidmixed off-gas to produce said aqueous ammonium carbamate solution.

4. The process of claim 1, in which said heat recovery zone of step (f)comprises said heat exchanger in which ammonium carbamate in said ureasynthesis eflluent is decomposed, and said fluid of step (t) is saidurea synthesis efiiuent.

5. The process of claim 1, in which ammonia is added to said resultinggaseous process stream removed according to step (e), prior to passingsaid gaseous process stream through said heat recovery zone, whereby theresulting combined process stream contains ammonia and carbon dioxide ina molar ratio between about 1.5 :1 to about 3:1.

6. The process of claim 5, in which ammonia is added to said resultinggaseous process stream in a proportion which produces a resultingcombined process stream with an ammonia to carbon dioxide molar ratio ofsubstantially 2: 1.

7. The process of claim 1, in which the molar ratio of ammonia to carbondioxide in said urea synthesis zone is in the range of 2:1 to about 8:1,the pressure in said urea synthesis zone is in the range of 135 kg./ sq.cm. to 270 kg./sq. cm., and the pressure of the effluent from the ureasynthesis zone is reduced to the range of about 15 kg./sq. cm. to 55kg./sq. cm.

8. The process of claim 1, in which said fluid passed to said heatrecovery zone according to step (f) is water, and said water isevaporated in said heat recovery zone to generate steam.

9. The process of claim 8, in which said generated steam is passed tosaid heat exchanger to heat said urea synthesis eflluent by indirectheat exchange and thereby decompose ammonium carbamate and generate saidmixed oif-gas.

10. The process of claim 1, in which said residual liquid effluentseparated from said mixed off-gas is processed to recover urea byreducing the pressure of said residual liquid eflluent, whereby a mixedvapor stream containing ammonia, carbon dioxide and water vapor isevolved from said residual liquid efiluent and the resulting liquidphase comprises an aqueous urea solution, and processing said evolvedmixed vapor stream to recover ammonia.

11. The process of claim 1, in which a liquid containing urea is addedto said resulting gaseous process stream removed according to step (e),prior to passing said gaseous process stream through said heat recoveryzone.

12. The process of claim 11, in which said added liquid containing ureais aqueous mother liquor solution derived from a crystallizer in whichsolid urea is crystallized from aqueous urea solution.

13. The process of claim 1, in which said mixed off-gas derived from thedecomposition ammonium carbamate in said urea synthesis effluent andcontaining ammonia, carbon dioxide and water vapor is at least partiallycondensed to produce said aqueous ammonium carbamate solution by coolingsaid mixed off-gas, whereby a portion of said mixed ofl-gas condenses tothe liquid state, scrubbing the residual mixed oft-gas with cold aqueousam moniacal ammonium carbamate solution, whereby carbon dioxide isabsorbed into said cold scrubbing solution, combining the resulting coldscrubbing solution with the condensed liquid portion derived from saidmixed ofl-gas, Withdrawing a portion of the resulting combined liquidsolution as said aqueous ammonium carbamate solution, and furthercooling and refluxing the residual mixed ottgas to produce ammonia gassubstantially free of carbon dioxide.

14. The process of claim 13, in which said mixed off-gas is initiallycooled by heat exchange with a feed stream of liquid ammonia, saidliquid ammonia stream being at a pressure in the range of kg./sq. cm. to350 kg./sq. cm., and the resulting warmed liquid ammonia stream ispassed to urea synthesis.

15. The process of claim 14, in which said warmed liquid ammonia streamis divided into a first portion and a second portion, said first portionis added to said resulting gaseous process stream removed according tostep (e), prior to passing said gaseous process stream through said heatrecovery zone, and said second portion is passed to said urea synthesiszone.

16. In a process for the synthesis of urea from ammonia and carbondioxide in a urea synthesis zone at an elevated pressure in the range of100 kg/sq. cm. to 350 kg./ sq. cm. in which the pressure of the effluentfrom the urea synthesis zone is reduced below 100 kg./sq. cm., the ureasynthesis eflluent is heated at reduced pressure in a heat exchanger todecompose ammonium carbamate and generate a mixed ofl-gas containingammonia, carbon dioxide and water vapor, the residual liquid effluent isseparated from said mixed off-gas and processed to recover product urea,and said mixed oil-gas is at least partially condensed to produce anaqueous ammonium carbamate solution containing free ammonia, theimproved process for recycle of ammonium carbamate to urea synthesiswhich comprises (a) pressurizing said aqueous ammonium carbamatesolution to an elevated urea synthesis pressure in the range of 100kg/sq. cm. to 350 kg./sq. cm.,

(b) passing said pressurized aqueous ammonium carbamate solution throughan externally heated stripping-heat exchange zone,

(c) passing a gaseous carbon dioxide stream through said stripping-heatexchange zone in contact with said aqueous ammonium carbamate solution,whereby ammonium carbamate is decomposed, ammonia and carbon dioxide arestripped from said solution into said gaseous carbon dioxide stream anda residual solution principally containing water is formed,

(d) removing said residual solution formed by step (c) from saidstripping-heat exchange zone,

(e) removing the resulting gaseous process stream formed by step (c)from said stripping-heat exchange zone, said resulting gaseous processstream containing ammonia and carbon dioxide components derived fromammonium carbamate decomposition in said zone together with carbondioxide derived from said gaseous carbon dioxide stream,

(f) adding ammonia to said resulting gaseous process stream removedaccording to step (e), whereby the resulting process stream containsammonia and carbon dioxide in a molar ratio between about 1.5 :1 toabout 3: 1,

(g) passing the resulting process stream formed by step (f) through saidheat exchanger in which ammonium carbamate in said urea synthesiseffluent is decomposed, whereby said resulting process stream passes inheat exchange with said urea synthesis effluent and at least a portionof said resulting process stream condenses to liquid ammonium carbamateand there- 13 by releases heat which is absorbed by said urea synthesisefiluent, and

(h) passing the resulting process stream discharged from said heatexchanger and containing condensed liquid ammonium carbamate to saidurea synthesis zone.

17. The process of claim 16, in which said strippingheat exchange zoneis vertically oriented, said pressurized aqueous ammonium carbamatesolution is passed downwards through said zone and said gaseous carbondioxide stream is passed upwards through said zone.

18. The process of claim 16, in which said residual solution principallycontaining water and removed according to step ((1) also contains aminor proportion of dissolved ammonia, said residual solution isdistilled to produce a gaseous stream containing ammonia, and saidgaseous stream is added to said mixed off-gas generated by decompositionof ammonium carbamate in said urea synthesis efiiuent, prior toprocessing said mixed off-gas to produce said aqueous ammonium carbamatesolution.

19. The process of claim 16, in which ammonia is added to said resultinggaseous process stream according to step (f) in a proportion whichproduces a resulting process stream with an ammonia to carbon dioxidemolar ratio of substantially 2:1.

20. The process of claim 16, in which the molar ratio of ammonia tocarbon dioxide in said urea synthesis zone is in the range of 2:1 toabout 821, the pressure in said urea synthesis zone is in the range of135 kg./sq. cm. to 270 kg./ sq. cm., and the pressure of the efiluentfrom the urea synthesis zone is reduced to the range of about kg./sq.cm. to 55 kg/sq. cm.

21. The process of claim 16, in which the resulting process streamcontaining condensed ammonium carbamate and formed by step (g) is passedthrough a heat recovery zone and in heat exchange with water, whereby afurther portion of said resulting process stream condenses to liquidammonium carbamate and thereby releases heat which is absorbed by saidwater to generate steam, prior to passing said resulting process streamcontaining condensed liquid ammonium carbamate to said urea synthesiszone.

22. The process of claim 16, in which said residual liquid efiluentseparated from said mixed off-gas is processed to recover urea byreducing the pressure of said residual liquid efiiuent, whereby a mixedvapor stream containing ammonia, carbon dioxide and water vaporisevolved from said residual liquid efiiuent and the resulting residualliquid phase comprises an aqueous urea solution, and processing saidevolved mixed vapor stream to recover ammonia.

23. The process of claim 16, in which a liquid containing urea is addedto said resulting gaseous process stream removed according to step (e),prior to passing said resulting process stream through said heatexchanger according to step (g).

24. The process of claim 23, in which said added liquid containing ureais aqueous mother liquor solution derived from a crystallizer in whichsolid urea is crystallized from aqueous urea solution.

25. The process of claim 16, in which said mixed offgas derived from thedecomposition of ammonium carbamate in said urea synthesis eflluent andcontaining ammonia, carbon dioxide and water vapor is at least partiallycondensed to produce said aqueous ammonium carbamate solution by coolingsaid mixed off-gas, whereby a portion of said mixed olT-gas condenses tothe liquid state, scrubbing the residual mixed elf-gas with cold aqueous"ammoniacal ammonium carbamate solution, whereby carbon dioxide isabsorbed into said cold scrubbing solution, combining the resulting coldscrubbing solution with the condensed liquid portion derived from saidmixed offgas, withdrawing a portion of the resulting combined liquidsolution as said aqueous ammonium carbamate solution, and furthercooling and refluxing the residual mixed olT-gas to produce ammonia gassubstantially free of carbon dioxide.

26. The process of claim 25, in which said mixed offgas is initiallycooled by heat exchange with a feed stream of liquid ammonia, saidliquid ammonia stream being at a pressure in the range of kg./ sq. cm.to 350 kg./sq. cm., and the resulting warmed liquid ammonia stream ispassed to urea synthesis.

27. The process of claim 26, in which said warmed liquid ammonia streamis divided into a first portion and a second portion, said first portionis added to said resulting gaseous process stream according to step (f),and said second portion is passed to said urea synthesis zone.

28. The process of claim 16, in which carbon dioxide is added to saidaqueous ammonium carbamate solution prior to step (b), whereby the addedcarbon dioxide reacts with free ammonia in said solution and formsfurther ammonium carbamate in solution and thereby produces an increasein the temperature of said aqueous ammonium carbamate solution.

References Cited UNITED STATES PATENTS 3,155,722 11/1964 Mavrovic260-555 3,232,983 2/1966 Flinn 260-555 3,248,425 4/1966 Ledergenber260'555 3,317,601 5/1967 Otsuka et a1 260-555 3,488,293 l/1970 Hong eta1. '260555 X BERNARD HELFIN, Primary Examiner M. W. GLYNN, AssistantExaminer

